This disclosure is directed to a diaphragm or membrane adapted to be placed in an alkali metal halide electrolysis cell. A classic example of this type of cell is the chlor-alkali cell. Briefly, the membrane defines adjacent anode and cathode chambers. Brine is introduced into the anode chamber and allowed to fill the cathode chamber by flowing through the diaphragm, thus completely wetting the diaphragm and flushing out all of the entrained air. An electric current flowing between anode and cathode in the two chambers creates ionic movement between the two chambers passing through the membrane. The process typically forms caustic and hydrogen discharge gas from the cathode side and chlorine gas on the anode side. The membrane is thus exposed to strong corrosive chemicals typically being caustic on one side and chlorine on the other. This occurs in the context of elevated temperatures and relatively substantial electric current densities across the membrane.
The traditionally used membranes are formed of asbestos fibers. A less popular membrane is one made of various fluoropolymers. An example is the fluoropolymer membrane manufactured by DuPont and sold under the trademark NAFION. There is probably a preponderance of membranes formed of asbestos. Asbestos is a material which is difficult to discard when the membrane is depleted or otherwise worn. Asbestos is an extremely difficult material to handle properly with environmental safety. It has the virtue of being operative at relatively high temperatures. It appears that the temperatures that asbestos can tolerate exceed those of NAFION membranes. As the temperature of the electrolytic cell is raised, the operating conditions become much more severe and the rate of wear or depletion is accelerated. Once an asbestos membrane has become depleted in operation, repair is unlikely and it simply must be discarded and replaced with a new membrane.
By contrast, the membrane in accordance with the teaching of this disclosure is one which can be used to depletion and thereafter recycled. That is, the membrane can be recycled as for instance by melting the binding resin. Alternatively, after grinding, the particles or fibers can be used as filler in plastic filled molded items. Another alternative is to recycle the spent membrane by combustion. Any of these approaches is sufficient to avoid simply placing the used membrane in some trash container. They avoid environmental difficulties arising from discarding asbestos membranes. Moreover, the membrane of this disclosure is adapted to operate at relatively high temperatures and in rigorous conditions where it is impervious to caustic or acids formed by or used in an electrolysis cell. Thermal stability appears to be higher than that of asbestos. Moreover, membranes formed by the disclosed process appear to have current efficiencies comparable with asbestos membranes. A significant amount of electrical power is required for a chlor-alkali cell as an example. The power consumption of a cell is indicated by the simple relationship of voltage drop times total cell current. The current is typically measured in ASI (amperes per square inch) or perhaps in amperes per square centimeter. A significant amount of power is applied to a cell and generates heat in an amount given by VI, referring to cell voltage drop and current. It is not uncommon to have a membrane which is several thousand square inches. With a cell voltage in the range of about two to three volts, and with a current density upwards of about 0.3 to even as high as 1.0 ASI, each square inch of the membrane contributes about 1 to 1.5 watts power to be dissipated by the cell. Clearly, with a membrane which is 20 feet long (240 inches or 6096 centimeters), the amount of heat liberated in the cell is quite high. The spot temperature at or adjacent to the membrane in the cell can climb quite high.
The membrane which is formed by the procedure of the present disclosure is able to handle such high temperatures with a marked degree of thermal stability. Two recent patents by deNora are perhaps representative of other attempts to solve the problem of avoiding an asbestos fiber membrane. U.S. Pat. Nos. 4,186,076 and 4,236,979 describe a fiberous porous matrix of various vinyls in conjuction with woven or unwoven carbon fiber felts. In similar fashion, U.S. Pat. No. 4,432,860 mentions fluoropolymers reinforced with carbon fibers. It appears that the references mentioned above and representative of the prior art do not disclose the procedure which is set forth below in detail. Briefly, a felt of carbon fibers having a specified range of lengths, and also having a range of diameters, is mixed with fluoropolymers of a specified formulation. The binder typically ranges from one to about 20 percent of membrane weight fluoropolymers, the optimum being about 10 to 15 percent. Utilizing a vacuum depositing procedure, the felt of fluoropolymers and carbon fibers is then dried and heated to the sinter point of the fluoropolymer whereupon the felt is formed. An optimum weight is in the range of about 0.4 to about 0.5 gm/in.sup.2 or about 0.1 gm/cm.sup.2. This yields a membrane which can handle current densities about up to 1.0 ASI. Moreover, it yields a membrane which is substantially nonconductive and yet which has a substantial surface area per unit weight.